The present invention relates to a propeller control system, and more particularly to an electronic/hydraulic control system for propeller blade angle control which minimizes the number of pressures which pass through a transfer bearing.
Common variable pitch propeller systems are actuated through metered hydraulic pressures generated in the stationary field of the engine and transferred into the rotational field of the propeller blades through a transfer bearing. Typically, hydraulic pressures required to adjust propeller blade pitch angle are supplied directly from the transfer bearing to a coarse pitch and a fine pitch chamber of the pitch change actuator to provide propeller pitch control. A multiple of additional pressures are also supplied through the transfer bearing to assure backup and feathering pressures which protect the propeller system against uncommanded blade angle excursions. Each of these pressures are separately communicated through the transfer bearing.
The transfer bearing must communicate the multiple of pressures from the stationary field to the rotational field while minimizing leakage. As the conventional transfer bearing supplies appropriately metered hydraulic pressure directly to each particular propeller pitch change system, any leakage may degrade the accuracy of the pitch change system. Conventional transfer bearings are therefore relatively complicated and critical systems. Moreover, the greater the number of pressures which must pass through the transfer bearing, the greater the complexity and expense thereof.
Accordingly, it is desirable to provide a propeller control system which minimizes the number of pressures which are communicated through a transfer bearing. It is further desirable to assure effective protection against uncommanded blade angle excursions while minimizing the overall system size, weight, complexity and expense.
The propeller control system according to the present invention provides for actuation through a supply pressure only. A transfer bearing thereby requires only a single land to provide supply pressure into the system for actuation and control. The supply pressure is metered at a pitch change valve within the rotating propeller shaft downstream of the transfer bearing. As the transfer bearing is upstream of the pitch change valve, leakage from the transfer bearing has minimal effect upon the accuracy of the system. Although only a supply pressure is output from the main pump other pressures are provided within the pitch change valve through particular orifice and valve sizing.
A main pump provides supply pressure to the transfer bearing through a pressure regulating valve. From the transfer bearing, supply pressure is provided to the pitch change valve. The pitch change valve translates to selectively communicate a coarse pitch change pressure and a fine pitch change pressure to supply pressure or drain pressure. Generally, by selectively communicating coarse pitch change pressure and fine pitch change pressure to supply pressure or drain pressure, speed governing, synchrophasing, beta control, feathering and unfeathering of the propeller blades are provided.
In the event of a failure, a backup mode of moving propeller blade angle in the coarse pitch direction (towards feather) is provided by the present invention. Movement towards the coarse pitch direction is the xe2x80x98fail-safexe2x80x99 action for propellers. A rotating controller and/or a separate non-rotating propeller backup control commands an increase in propeller blade angle if at least any of the following conditions are detected: a) propeller overspeed condition, b) propeller blade angle below the minimum in-flight value during a flight, or c) receipt of a feather command from the cockpit.
The command to increase the propeller blade angle is an electrical signal that energizes a feathering solenoid. Actuation of the feathering solenoid increases the supply pressure within the system.
The increased supply pressure changes the hydraulic force balance of the pitch change valve to causes a feather actuating valve to move forward relative a pitch change valve sleeve until a feather override orifice between supply pressure and a Feather Override Chamber is closed. The Feather Override Chamber is connected to drain pressure and the pressure within the Feather Override Chamber drops to drain pressure. The balance of forces will be rapidly changed causing the Pitch Change Valve Spool to translate aft of its starting position. In response, a coarse pitch pressure metering window is opened to allow fluid at the increased supply pressure into the Coarse Pitch chamber while simultaneously opening a Fine Pitch Pressure Metering Window to allow fluid within a Fine Pitch chamber to flow to drain pressure. The flow to drain changes the hydraulic balance on a Pitch Change Actuator Piston and drives a pitch link attached to each propeller blade to thereby change the pitch thereof towards coarse pitch. The feather override mode will be maintained until either a) the pitch change actuator piston actuator reaches the feather position and contacts a mechanical feather stop, or b) the controller de-energizes the feathering solenoid because the hazardous condition no longer exists.
Accordingly, the present invention provides a propeller control system which minimizes the number of pressures which are communicated through a transfer bearing, assures effective protection against uncommanded blade angle excursions, and reduces the overall system size, weight, complexity and expense.